In boundary lubrication, surface asperities often come in contact with each other even though the lubricant present between those surfaces serves to carry much of the existing load. Extreme pressure (EP) additives are a special class of boundary lubrication additives which chemically react with the metal surface to form compounds with lower shear strength than the metal. The resultant low-shear compound thus provides the requisite lubrication. EP oils are basically inhibited oils with added extreme pressure additives. The EP agent serves to control wear in the boundary lubrication phase; namely, starting stopping, shock loading and the like. If high points of mating surfaces come in contact during machine operation, the lower shear strength EP compound will shear, rather than fuse and cause scoring; thus, controlled wear is exchanged for destructive wear. EP additives find utility in greases, industrial oils and gear lubes.
Organic sulfur compounds have been known to have utility as additives for lubricating oils. These compounds provide extreme pressure properties to lubricants especially under high speed shock conditions. Unfortunately, the presence of sulfur in lubricating oils causes considerable corrosion of metals, particularly copper. Since lubricating oils often operate at relatively high temperatures, thermally unstable sulfur compounds may break down resulting in loss of the extreme pressure property and in increased metal corrosion. In U.S. Pat. No. 2,708,199, there is disclosed a method of producing organic polysulfides from olefins having from six to 30 carbon atoms resulting in polymers of the olefin containing two to three sulfur atoms per unsaturated bond of the olefin. However, without proper control of the reaction conditions, the resulting compound may be highly corrosive and unstable. Moreover, if olefins of less than six carbon atoms were used in this process, oil insoluble products would be obtained.
U.S. Pat. Nos. 3,471,404 and 3,697,499, the contents of which are incorporated by reference in their entirety, disclose stable oil-soluble organic sulfides having extreme pressure properties and low corrosiveness to metal. These patents teach that lubricating oil compositions containing an effective amount of the additives disclosed therein evidence good load carrying capability. Sulfurized isobutylene is one example of such an oil-soluble organic sulfides having extreme pressure properties.
U.S. Pat. No. 4,317,738 discloses that improved dispersants can be prepared by oxidizing an olefin and reacting the oxidized olefin with sulfur or a sulfur-yielding compound and an amine. The olefin can have a molecular weight of from about 150 to 140,000, but preferably ranges from about 300 to 100,000. The dispersants disclosed are said to have utility in lubricant compositions in amounts of about 0.1 to 10 percent based on the oil.
While sulfurized isobutylene has been found to be an effective EP additive, finding acceptance in formulations where such service is required, sulfurized isobutylene exhibits a specific low fatigue life, as measured by the well-known tapered roller bearing test. It would be desirable in many applications to use an EP additive which could exhibit an increase in fatigue life in the tapered roller bearing test.
Therefore what is needed is an EP additive which is both effective in extreme pressure applications and demonstrates an increase in fatigue life.